Processes for purifying acetic and hydrating anhydride

ABSTRACT

Processes for purifying acetic acid by distilling a process in a column in which acetic anhydride is formed in the lower portion of the column. The product stream withdrawn from the column comprises acetic acid, water at a concentration of no more than 0.2 wt. %, and acetic anhydride at a concentration of no more than 600 wppm. The process further comprises hydrating the acetic anhydride in the product stream to form a purified acetic acid product comprising acetic anhydride at a concentration of no more than 50 wppm.

CROSS-REFERENCES TO RELATED APPLICATIONS

This patent application claims the priority of U.S. Application No.62/255,060, filed Nov. 13, 2015, the disclosure of which is incorporatedherein by reference in its entirety.

FIELD OF THE INVENTION

This invention relates to processes for producing acetic acid and, inparticular, to improved processes for hydrating acetic anhydride in aproduct stream to form a purified acetic acid product comprising aceticanhydride at a concentration of no more than 50 wppm.

BACKGROUND OF THE INVENTION

Among currently employed processes for synthesizing acetic acid, one ofthe most useful commercially is the catalyzed carbonylation of methanolwith carbon monoxide as taught in U.S. Pat. No. 3,769,329, incorporatedherein by reference in its entirety. The carbonylation catalyst containsrhodium, either dissolved or otherwise dispersed in a liquid reactionmedium or supported on an inert solid, along with a halogen-containingcatalyst promoter as exemplified by methyl iodide. The rhodium can beintroduced into the reaction system in any of many forms. Likewise,because the nature of the halide promoter is not generally critical, alarge number of suitable promoters, most of which are organic iodides,may be used. Most typically and usefully, the reaction is conducted bycontinuously bubbling carbon monoxide gas through a liquid reactionmedium in which the catalyst is dissolved.

A widely used and successful commercial process for synthesizing aceticacid involves the catalyzed carbonylation of methanol with carbonmonoxide. The catalyst contains rhodium and/or iridium and a halogenpromoter, typically methyl iodide. The reaction is conducted bycontinuously bubbling carbon monoxide through a liquid reaction mediumin which the catalyst is dissolved. The reaction medium comprises aceticacid, methyl acetate, water, methyl iodide and the catalyst. Commercialprocesses for the carbonylation of methanol include those described inU.S. Pat. No. 3,769,329. Another conventional methanol carbonylationprocess includes the Cativa™ process, which is discussed in Jones, J. H.(2002), “The Cativa™ Process for the Manufacture of Acetic Acid,”Platinum Metals Review, 44 (3): 94-105, the entirety of which isincorporated herein by reference.

The AO™ process for the carbonylation of an alcohol to produce thecarboxylic acid having one carbon atom more than the alcohol in thepresence of a rhodium catalyst is disclosed in U.S. Pat. Nos. 5,001,259;5,026,908; and 5,144,068; and EP0161874, the entireties of which areincorporated herein by reference. As disclosed therein, acetic acid isproduced from methanol in a reaction medium containing methyl acetate(MeAc), methyl halide, especially methyl iodide (MeI), and rhodiumpresent in a catalytically effective concentration. These patentsdisclose that catalyst stability and the productivity of thecarbonylation reactor can be maintained at high levels, even at very lowwater concentrations, i.e., 4 weight percent or less, (despite the priorpractice of maintaining approximately 14-15 wt. % water) by maintainingin the reaction medium, along with a catalytically effective amount ofrhodium, at least a finite concentration of water, e.g., 0.1 wt. %, anda specified concentration of iodide ions over and above the iodide ionthat is present as hydrogen iodide. This iodide ion is a simple salt,with lithium iodide being preferred. The salt may be formed in situ, forexample, by adding lithium acetate, lithium carbonate, lithium hydroxideor other lithium salts of anions compatible with the reaction medium.The patents teach that the concentration of methyl acetate and iodidesalts are significant parameters in affecting the rate of carbonylationof methanol to produce acetic acid, especially at low reactor waterconcentrations. By using relatively high concentrations of the methylacetate and iodide salt, a high degree of catalyst stability and reactorproductivity is achieved even when the liquid reaction medium containswater in finite concentrations as low as 0.1 wt. %. Furthermore, thereaction medium employed improves the stability of the rhodium catalyst,i.e., resistance to catalyst precipitation, especially during theproduct recovery steps of the process. In these steps, distillation forthe purpose of recovering the acetic acid product tends to remove fromthe catalyst the carbon monoxide, which in the environment maintained inthe reaction vessel, is a ligand with stabilizing effect on the rhodium.

U.S. Pat. No. 5,144,068 discloses a process for producing acetic acid byreacting methanol with carbon monoxide in a liquid reaction mediumcontaining a rhodium (Rh) catalyst and comprising water, acetic acid,methyl iodide, and methyl acetate, wherein catalyst stability ismaintained in the reaction by maintaining in said reaction medium duringthe course of said reaction 0.1 wt. % to 14 wt. % of water together with(a) an effective amount in the range of 2 wt. % to 20 wt. % of acatalyst stabilizer selected from the group consisting of iodide saltswhich are soluble in said reaction medium in effective concentration atreaction temperature, (b) 5 wt. % to 20 wt. % of methyl iodide, and (c)0.5 wt. % to 30 wt. % of methyl acetate. Suitable iodide salts may be aquaternary iodide salt or an iodide salt of a member of the groupconsisting of the metals of Group IA and Group IIA of the PeriodicTable.

Carbonyl impurities, such as acetaldehyde, that are formed during thecarbonylation of methanol may react with iodide catalyst promoters toform multi-carbon alkyl iodides, e.g., ethyl iodide, propyl iodide,butyl iodide, pentyl iodide, hexyl iodide, and the like. It is desirableto remove multi-carbon alkyl iodides from the reaction product becauseeven small amounts of these impurities in the acetic acid product tendto poison the catalyst used in the production of vinyl acetate, aproduct commonly produced from acetic acid.

Conventional techniques to remove such impurities include treating thecrude acid product streams with oxidizers, ozone, water, methanol,activated-carbon, amines, and the like. Such treatments may or may notbe combined with distillation of the acetic acid. The most typicalpurification treatment involves a series of distillations to yield asuitable purified acetic acid as the final product. It is also known toremove carbonyl impurities from organic streams by treating the organicstreams with an amine compound such as hydroxylamine, which reacts withthe carbonyl compounds to form oximes, followed by distillation toseparate the purified organic product from the oxime reaction products.However, the additional treatment of the purified acetic acid adds costto the process, and distillation of the treated acetic acid product canresult in additional impurities being formed.

While it is possible to obtain acetic acid of relatively high purity,the acetic acid product formed by the low-water carbonylation processand purification treatment described above frequently remains somewhatdeficient with respect to the permanganate time due to the presence ofsmall proportions of residual impurities. Because a sufficientpermanganate time is an important commercial test, which the acidproduct may be required to meet to be suitable for many uses, thepresence of impurities that decrease permanganate time is objectionable.Moreover, it has not been economically or commercially feasible toremove minute quantities of these impurities from the acetic acid bydistillation because some of the impurities have boiling points close tothat of the acetic acid product or halogen-containing catalystpromoters, such as methyl iodide. It has thus become important toidentify economically viable methods of removing impurities elsewhere inthe carbonylation process without contaminating the purified acetic acidor adding unnecessary costs.

Macroreticulated or macroporous strong acid cationic exchange resincompositions are conventionally utilized to reduce iodide contamination.Suitable exchange resin compositions, e.g., the individual beadsthereof, comprise both sites that are functionalized with a metal, e.g.,silver, mercury or palladium, and sites that remain in the acid form.Exchange resin compositions that have little or no metal-functionalitydo not efficiently remove iodides and, as such, are not conventionallyused to do so. Typically, metal-functionalized exchange resins areprovided in a fixed bed and a stream comprising the crude acetic acidproduct is passed through the fixed resin bed. In the metalfunctionalized resin bed, the iodide contaminants contained in the crudeacetic acid product are removed from the crude acid product stream.

Widely used and successful commercial processes for synthesizing aceticanhydride also involves the catalyzed carbonylation of methanol withcarbon monoxide. Acetic anhydride processes have been disclosed in U.S.Pat. Nos. 5,292,948; 4,374,070; 4,115,444; and 4,046,807, the entiretiesof which are incorporated herein by reference.

Other ion exchange resins have been used to remove iodide impuritiesfrom acetic acid and/or acetic anhydride. U.S. Pat. No. 6,657,078describes a low-water process that uses a metal-functionalized exchangeresin to remove iodides. The reference also avoids the use of a heavyends column, resulting in energy savings. U.S. Pat. No. 5,220,058 alsodiscloses the use of ion exchange resins having metal exchanged thiolfunctional groups for removing iodide impurities from acetic acid and/oracetic anhydride. Typically, the thiol functionality of the ion exchangeresin has been exchanged with silver, palladium, or mercury. U.S. Pat.No. 5,227,524 discloses a process for removing iodide derivatives fromliquid acetic acid and/or acetic anhydride comprises contacting theliquid acetic acid and/or acetic anhydride with a strong acid cationexchange resin having from about 4% to about 12% crosslinking, a surfacearea in the proton exchanged form of less than 10 m²g⁻¹ after dryingfrom the water wet state and a surface area of greater than 10 m²g⁻¹after drying from a wet state in which water has been replaced bymethanol. The resin has at least one percent of its active sitesconverted to the silver form, preferably from 30 to 70 percent.

U.S. Pat. No. 5,801,279 discloses a method which can reduce the amountof silver or mercury dissolved in a solution after contact and canincrease the usage of silver or mercury without installing new treatingfacilities in a process for removing iodine compounds contained in anorganic medium, particularly acetic acid or a mixture of acetic acid oracetic anhydride, by contacting them with a cation exchange resin inwhich at least 1% of the active sites are converted to a silver form ora mercury form. This disclosed method is characterized by carrying outthe operation while elevating the temperatures in stages whilecontacting the organic medium, particularly acetic acid or a mixture ofacetic acid and acetic anhydride, containing the iodine compounds with acation exchange resin.

U.S. Pat. No. 5,344,976 discloses that the metal ion contaminants in theacid and/or anhydride may arise from corrosion or the use of reagents inthe upstream process. The patent describes the use of a cationicexchanger in the acid form to remove at least a portion of the metal ioncontaminants such as iron, potassium, calcium, magnesium, and sodiumfrom a carboxylic acid stream prior to contacting the stream with theexchanged strong acid cation exchange resin to remove C₁ to C₁₀ alkyliodide compounds, hydrogen iodide or iodide salts.

U.S. Pat. No. 5,648,531 discloses a process for continuously producingacetic anhydride alone or acetic anhydride and acetic acid by reactingmethyl acetate and/or dimethyl ether and, optionally, water and/ormethanol, with carbon monoxide alone or carbon monoxide and hydrogen inthe presence of a rhodium compound and methyl iodide as principalcatalysts. Trace impurities causative of tar formation are distilled andseparated in an evaporator and/or a subsequent refining step to removethe same. The removal of the trace impurities causative of tar formationserves to decrease the amount of tar formed as an impurity.

U.S. Pat. No. 8,759,576 discloses a process for purifying aceticanhydride. The process includes the steps of feeding a liquid crudeacetic anhydride stream directly to a distillation column and separatingthe liquid crude acetic anhydride stream to produce a light ends stream,a sidedraw and a residue stream. The sidedraw comprises substantiallypure acetic anhydride product. The distillation column is operated at apressure less than 101 kPa. The substantially pure acetic anhydrideproduct comprises greater than 98 wt. % acetic anhydride, has apermanganate time of greater than 10 minutes, and has an APHA color ofless than 10.

While the above-described processes have been successful, the needexists for improved processes for producing acetic acid, in particular,for methods for removing acetic anhydride from those processes.

SUMMARY OF THE INVENTION

This invention generally relates to processes for the production ofacetic acid. One embodiment of the present invention relates to aprocess for purifying acetic acid, comprising distilling a processstream in a column, the process stream comprising acetic acid at aconcentration of greater than 90 wt. %, water at a concentration from 1to 3 wt. %, one or more C₁-C₁₄ alkyl iodides in a total concentration ofno more than 6 wt. %, and methyl acetate at a concentration of no morethan 6 wt. %, forming acetic anhydride in a lower portion of the column,withdrawing a product stream from the lower portion of the column, theproduct stream comprising acetic acid, water at a concentration of nomore than 0.2 wt. %, and acetic anhydride at a concentration of no morethan 600 wppm, e.g., from 5 to 600 wppm, and hydrating the aceticanhydride in the product stream to form a purified acetic acid productcomprising acetic anhydride at a concentration of no more than 50 wppm,e.g., from 0.5 to 50 wppm. Preferably, the acetic anhydrideconcentration in the purified acetic acid product is less than theacetic anhydride concentration in the product stream. In one embodiment,the water concentration is maintained at a concentration of no more than0.2 wt. %. The hydrating step may reduce the acetic anhydrideconcentration by at least 60%. In one embodiment, the hydrating stepcomprises contacting the product stream with a cation exchange resin.The cation exchange resin may comprise sulfonic acid or partially metalimpregnated sulfonic acid functional groups. In one embodiment, theproduct stream is not contacted with an aqueous stream after withdrawalof the product stream from the lower portion of the column. The processstream that is distilled is substantially free of acetic anhydride,e.g., the acetic anhydride concentrations are below detectable limits.In addition to acetic acid and water, the process stream may alsocomprise hydrogen iodide at a concentration of no more than 300 wppm.The product stream may be withdrawn at a point within 5 actual stagesfrom the base of the column. The water concentration in the lowerportion of the column is maintained to be less than the waterconcentration of the process stream. In further embodiments, the processcomprises contacting the purified acetic acid product with ametal-exchanged ion exchange resin having acid cation exchange sites toreduce the iodide concentration of the purified acetic acid product tono more than 100 wppb.

In another embodiment, there is provided a process for purifying aceticacid comprising carbonylating, in a reactor, at least one memberselected from the group consisting of methanol, dimethyl ether, andmethyl acetate in a reaction medium comprising water at a concentrationfrom 0.1 to 14 wt. %, a rhodium catalyst, methyl iodide, and an iodidesalt, separating the reaction medium to form a liquid recycle stream anda vapor product stream, distilling at least a portion of the vaporproduct stream in a first column to obtain a side stream comprisingacetic acid at a concentration greater than 90 wt. %, water at aconcentration from 1 to 3 wt. %, one or more C₁-C₁₄ alkyl iodides in atotal concentration of no more than 6 wt. % and methyl acetate at aconcentration of no more than 6 wt. %, distilling the side stream in asecond column to obtain a product stream comprising acetic acid, waterat a concentration of no more than 0.2 wt. % and acetic anhydride at aconcentration of no more than 600 wppm, e.g., from 10 to 600 wppm, andcontacting the product stream with a cation exchange resin to form apurified acetic acid product comprising no more than 50 wppm aceticanhydride. In one embodiment, the production rate of acetic anhydride isgreater in the second column than the reactor. The side stream issubstantially free of acetic anhydride, e.g., the acetic anhydrideconcentrations are below detectable limits. The side stream may alsocomprise hydrogen iodide at a concentration of no more than 300 wppm.The purified acetic acid product may comprise water at a concentrationof no more than 0.2 wt. % and acetic anhydride at a concentration of nomore than 10 wppm. In one embodiment, the water concentration in a lowerportion of the second column is maintained to be less than the waterconcentration of the side stream. The acetic anhydride concentration ofthe purified acetic acid product is less than the acetic anhydrideconcentration of the product stream. The cation exchange resin mayreduce the acetic anhydride concentration of the product stream by atleast 60%. In further embodiments, the process comprises contacting thepurified acetic acid product with a metal-exchanged ion exchange resinhaving acid cation exchange sites to reduce the iodide concentration ofthe purified acetic acid product to no more than 100 wppb.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood in view of the appendednon-limiting figures, wherein:

FIG. 1 illustrates a process for purifying acetic acid with cationexchange resin to hydrate acetic anhydride and a metal functionalizedfixed resin bed for iodide removal.

FIG. 2 illustrates another process for purifying acetic acid from asidedraw with cation exchange resin to hydrate acetic anhydride and ametal functionalized fixed resin bed for iodide removal.

DETAILED DESCRIPTION OF THE INVENTION

At the outset, it should be noted that in the development of any suchactual embodiment, numerous implementation—specific decisions must bemade to achieve the developer's specific goals, such as compliance withsystem related and business related constraints, which will vary fromone implementation to another. In addition, the processes disclosedherein can also comprise components other than those cited orspecifically referred to, as is apparent to one having average orreasonable skill in the art.

In the summary and this detailed description, each numerical valueshould be read once as modified by the term “about” (unless alreadyexpressly so modified), and then read again as not so modified unlessotherwise indicated in context. Also, in the summary and this detaileddescription, it should be understood that a concentration range listedor described as being useful, suitable, or the like, is intended thatany and every concentration within the range, including the end points,is to be considered as having been stated. For example, a range “from 1to 10” is to be read as indicating each and every possible number alongthe continuum between about 1 and about 10. Thus, even if specific datapoints within the range, or even no data points within the range, areexplicitly identified or refer to only a few specific data points, it isto be understood that inventors appreciate and understand that any andall data points within the range are to be considered to have beenspecified, and that inventors possessed knowledge of the entire rangeand all points within the range.

Throughout the entire specification, including the claims, the followingterms have the indicated meanings unless otherwise specified.

As used in the specification and claims, “near” is inclusive of “at.”The term “and/or” refers to both the inclusive “and” case and theexclusive “or” case, and is used herein for brevity. For example, amixture comprising acetic acid and/or methyl acetate may comprise aceticacid alone, methyl acetate alone, or both acetic acid and methylacetate.

All percentages are expressed as weight percent (wt. %), based on thetotal weight of the particular stream or composition present, unlessotherwise noted. Room temperature is 25° C. and atmospheric pressure is101.325 kPa unless otherwise noted.

For purposes herein: acetic acid may be abbreviated as “AcOH”;acetaldehyde may be abbreviated as “AcH”; methyl acetate may beabbreviated “MeAc”; methanol may be abbreviated “MeOH”; methyl iodidemay be abbreviated as “MeI”; hydrogen iodide may be abbreviated as “HI”;carbon monoxide may be abbreviated “CO”; and dimethyl ether may beabbreviated “DME”.

HI refers to either molecular hydrogen iodide or dissociated hydriodicacid when at least partially ionized in a polar medium, typically amedium comprising at least some water. Unless otherwise specified, thetwo are referred to interchangeably. Unless otherwise specified, HIconcentration is determined via acid-base titration using apotentiometric end point. In particular, HI concentration is determinedvia titration with a standard lithium acetate solution to apotentiometric end point. It is to be understood that for purposesherein, the concentration of HI is not determined by subtracting aconcentration of iodide assumed to be associated with a measurement ofcorrosion metals or other non H+ cations from the total ionic iodidepresent in a sample.

It is to be understood that HI concentration does not refer to iodideion concentration. HI concentration specifically refers to HIconcentration as determined via potentiometric titration.

This subtraction method is an unreliable and imprecise method todetermine relatively lower HI concentrations (i.e., less than about 5weight percent) due to the fact that it assumes all non-H+ cations (suchas cations of Fe, Ni, Cr, Mo) are associated with iodide anionexclusively. In reality, a significant portion of the metal cations inthis process can be associated with acetate anion. Additionally, many ofthese metal cations have multiple valence states, which adds even moreunreliability to the assumption on the amount of iodide anion whichcould be associated with these metals. Ultimately, this method givesrise to an unreliable determination of the actual HI concentration,especially in view of the ability to perform a simple titration directlyrepresentative of the HI concentration.

For purposes herein, an “overhead” or “distillate” of a distillationcolumn refers to at least one of the lower boiling condensable fractionswhich exits at or near the top, (e.g., proximate to the top), of thedistillation column, and/or the condensed form of that stream orcomposition. Obviously, all fractions are ultimately condensable, yetfor purposes herein, a condensable fraction is condensable under theconditions present in the process as readily understood by one of skillin the art. Examples of noncondensable fractions may include nitrogen,hydrogen, and the like. Likewise, an overhead stream may be taken justbelow the upper most exit of a distillation column, for example, whereinthe lowest boiling fraction is a non-condensable stream or represents ade-minimis stream, as would be readily understood by one of reasonableskill in the art.

The “bottoms” or “residuum” of a distillation column refers to one ormore of the highest boiling fractions which exit at or near the bottomof the distillation column, also referred to herein as flowing from thebottom sump of the column. It is to be understood that a residuum may betaken from just above the very bottom exit of a distillation column, forexample, wherein the very bottom fraction produced by the column is asalt, an unusable tar, a solid waste product, or a de-minimis stream aswould be readily understood by one of reasonable skill in the art.

For purposes herein, distillation columns comprise a distillation zoneand a bottom sump zone. The distillation zone includes everything abovethe bottom sump zone, i.e., between the bottom sump zone and the top ofthe column. For purposes herein, the bottom sump zone refers to thelower portion of the distillation column in which a liquid reservoir ofthe higher boiling components is present (e.g., the bottom of adistillation column) from which the bottom or residuum stream flows uponexiting the column. The bottom sump zone may include reboilers, controlequipment, and the like.

It is to be understood that the term “passages,” “flow paths,” “flowconduits,” and the like in relation to internal components of adistillation column are used interchangeably to refer to holes, tubes,channels, slits, drains, and the like, which are disposed through and/orwhich provide a path for liquid and/or vapor to move from one side ofthe internal component to the other side of the internal component.Examples of passages disposed through a structure such as a liquiddistributor of a distillation column include drain holes, drain tubes,drain slits, and the like, which allow a liquid to flow through thestructure from one side to another.

Average residence time is defined as the sum total of all liquid volumehold-up for a given phase within a distillation zone divided by theaverage flow rate of that phase through the distillation zone. Thehold-up volume for a given phase can include liquid volume contained inthe various internal components of the column including collectors,distributors and the like, as well as liquid contained on trays, withindowncomers, and/or within structured or random packed bed sections.

Hydration of Acetic Anhydride

The present invention relates to processes for the purification ofacetic acid and, in particular, to improved processes for hydratingacetic anhydride in a product stream comprising acetic acid, water at aconcentration of no more than 0.2 wt. %, and acetic anhydride at aconcentration of no more than 600 wppm. Advantageously, the disclosedembodiments provide a purified acetic acid product comprising aceticanhydride at a concentration of no more than 50 wppm, e.g., no more than40 wppm, no more than 30 wppm, no more than 20 wppm, no more than 10wppm or no more than 5 wppm. In terms of ranges, the purified aceticacid product comprises acetic anhydride at a concentration from 0.5 to50 wppm, e.g., from 0.5 to 40 wppm, from 0.5 to 30 wppm, from 0.5 to 20wppm, or from 0.5 to 10 wppm. The purified acetic acid product comprisesless acetic anhydride than the product stream. This process yields ahigh quality purified acetic acid product that may be widely used invarious applications.

During purification of acetic acid obtained by methanol carbonylation,one or more distillation columns may be used to separate impurities fromthe process stream and yield a purified product stream according toembodiments of the present invention. Typically, in one of thedistillation columns, water is separated to yield a glacial acetic acidproduct having a water concentration of no more than 0.2 wt. %, e.g., nomore than 0.15 wt. %, no more than 0.1 wt. %, or no more than 0.05 wt.%. It has been found that in the lower portion of this column, due tothe substantially anhydrous conditions (no more than 0.2 wt. % water),acetic anhydride side reactions may occur, leading to acetic anhydrideformation that contaminates the product stream. This may lead todeleterious acetic acid products that have quality control issues.Although acetic acid/acetic anhydride co-production may involveseparating these components, it is not practical or efficient toseparate acetic anhydride from the product stream using such methods.Advantageously, the present invention overcomes problems associated withthe acetic anhydride contamination by hydrating the product to reduceand/or eliminate acetic anhydride.

In one embodiment, the process stream fed to the distillation columns issubstantially free of acetic anhydride, meaning the acetic anhydrideconcentrations are below detectable limits. Detectable limits may detectacetic anhydride in concentration of greater than 0.5 wppm. Stateddifferently, the process stream is obtained from a methanolcarbonylation process that produces acetic acid under aqueousconditions, e.g., in a reaction medium comprising water at aconcentration of greater than 0.1 wt. %. Thus, the process stream issubstantially free of acetic anhydride produced in the carbonylationreactor.

In one embodiment, there is provided a process for purifying aceticacid, comprising distilling a process stream in a column, the processstream comprising acetic acid and water, wherein the process stream issubstantially free of acetic anhydride, forming acetic anhydride in alower portion of the column, withdrawing a product stream from the lowerportion of the column, the product stream comprising acetic acid, waterat a concentration of no more than 0.2 wt. %, and acetic anhydride at aconcentration of no more than 600 wppm, and hydrating the aceticanhydride in the product stream to form a purified acetic acid productcomprising acetic anhydride at a concentration of no more than 50 wppm.

The process streams that are separated comprises acetic acid, water, andother components, such as, but not limited to methyl iodide, methylacetate, hydrogen iodide, acetaldehyde, and propionic acid. Variousprocess streams are disclosed herein. One exemplary process streamcomprises acetic acid in amount of greater than or equal to 90 wt. %,e.g., greater than or equal to 94 wt. % or greater than or equal to 96wt. %. The water concentration of the process stream may be in an amountfrom 1 to 3 wt. %, e.g., preferably from 1 to 2.5 wt. % and morepreferably from 1.1 to 2.1 wt. %. The process stream may also compriseone or more C₁-C₁₄ alkyl iodides in a total concentration of no morethan 6 wt. %, e.g., no more than 4 wt. %, or no more than 3.6 wt. %, andmethyl acetate at a concentration of no more than 6 wt. %, e.g., no morethan 4 wt. %, or no more than 3.6 wt. %. In some embodiments, inaddition to acetic acid and water, the exemplary process stream may alsocomprise one or more C₁-C₁₄ alkyl iodides in an amount from 0.1 to 6 wt.%, e.g., from 0.5 to 5 wt. %, from 0.6 to 4 wt. %, from 0.7 to 3.7 wt.%, or from 0.8 to 3.6 wt. %. Generally, methyl iodide is the primaryalkyl iodide and the concentration of the one or more C₁-C₁₄ alkyliodides may be determined by the methyl iodide concentration. Due to thepresence of water, the process stream may also contain methyl acetate inan amount from 0.1 to 6 wt. %, e.g., from 0.5 to 5 wt. %, from 0.6 to 4wt. %, from 0.7 to 3.7 wt. %, or from 0.8 to 3.6 wt. %. In someembodiments, the process stream may also comprise hydrogen iodide at aconcentration of no more than 300 wppm, e.g., or no more than 250 wppm,no more than 200 wppm, no more than 100 wppm, no more than 50 wppm, nomore than 25 wppm, or no more than 10 wppm.

In one embodiment, there is provided a process for purifying acetic acidcomprising distilling a process stream in a column, the process streamcomprising acetic acid at a concentration greater than 90 wt. %, waterat a concentration from 1 to 3 wt. %, one or more C₁-C₁₄ alkyl iodidesin a total concentration of no more than 6 wt. % (from 0.1 to 6 wt. %),and methyl acetate at a concentration of no more than 6 wt. % (from 0.1to 6 wt. %), and optionally hydrogen iodide at a concentration of nomore than 300 wppm, forming acetic anhydride in a lower portion of thecolumn, withdrawing a product stream from the lower portion of thecolumn, the product stream comprising acetic acid, water at aconcentration of no more than 0.2 wt. %, and acetic anhydride at aconcentration of no more than 600 wppm, and hydrating the aceticanhydride in the product stream to form a purified acetic acid productcomprising acetic anhydride at a concentration of no more than 50 wppm.

In one embodiment, the distillation column separates a process stream toyield a product stream comprising acetic acid, water, and aceticanhydride. The presence of acetic anhydride in the product stream isundesirable. In one embodiment, the product stream comprises acetic acidat a concentration of greater than or equal to 99.5 wt. %, e.g., greaterthan or equal to 99.7 wt. % or greater than or equal to 99.9 wt. %. Theproduct stream comprises water at a concentration of no more than 0.2wt. %, e.g., no more than 0.15 wt. %, no more than 0.1 wt. %, or no morethan 0.05 wt. %. The product stream comprises a concentration of aceticanhydride that is undesirable and causes product quality issues. Evenlow amounts of acetic anhydride, e.g., amounts of 5 wppm, may contributeto product quality issues. In one embodiment, the product streamcomprises acetic anhydride at a concentration of no more than 600 wppm,e.g., no more than 500 wppm, no more than 450 wppm, no more than 400wppm, no more than 300 wppm, no more than 200 wppm, no more than 100wppm, or no more than 50 wppm. In terms of ranges, the product streamcomprises acetic anhydride in an amount from 5 to 600 wppm, e.g., from10 to 600 wppm, from 5 to 450 wppm, from 10 to 450 wppm, from 10 to 300wppm, or from 10 to 100 wppm. The embodiments of the present inventionmay also be used to reduce acetic anhydride in the product stream whenthe concentration exceeds 600 wppm. To treat these higher concentrationsof acetic anhydride, the hydration may be repeated one or more times. Ofcourse, if acetic anhydride is introduced into the column, the aceticanhydride may be higher than when it is not added, and the presentinvention may also remove the added acetic anhydride.

The product stream may be withdrawn from a location of a distillationcolumn that yields a product stream having a water concentration that isless than the water concentration in the process stream. It has beenfound that in this portion of the distillation column, that even if noacetic anhydride is introduced into the column, acetic anhydride may beundesirably formed in the presence of substantially anhydrousconditions, e.g. no more than 0.2 wt. %. For example, the product streammay be withdrawn from the base of a distillation column or as a sidedrawfrom a lower portion of the distillation. In one embodiment, thesidedraw is withdrawn within 5 actual stages of the base, e.g., within 4actual stages, within 3 actual stages, within 2 actual stages, within 1actual stage, and preferably above the base of the second column. Thesecond column may have from 10 to 80 actual states, e.g., from 15 to 80actual stages or from 20 to 80 actual stages. An actual stage maycorrespond to a plate in a column. The sidedraw may be a liquid or vaporsidedraw.

In one embodiment, there is provided a process for purifying aceticacid, comprising distilling a process stream in a column, the processstream comprising acetic acid and water, wherein the process stream issubstantially free of acetic anhydride, forming acetic anhydride in alower portion of the column, withdrawing a product stream from the lowerportion of the column, the product stream comprising acetic acid, waterat a concentration of no more than 0.2 wt. %, and acetic anhydride at aconcentration of no more than 600 wppm, and contacting the productstream with a cation exchange resin to hydrate the acetic anhydride inthe product stream to form a purified acetic acid product comprisingacetic anhydride at a concentration of no more than 50 wppm.

In one embodiment, the hydration of acetic anhydride may be conducted bycontacting the product stream with a cation exchange resin. The productstream may be withdrawn from the distillation column contacted and withthe cation exchange resin. Acetic anhydride is reacted in the cationexchange resin through a hydration reaction to convert the aceticanhydride into acetic acid. Because water has already been removed fromthe product stream, it is not desirable to increase the waterconcentration in the cation exchange resin. Thus, no aqueous streams arecontacted with the product stream after withdrawal from the lowerportion. In one embodiment, the water concentration in the cationexchange resin is maintained at or below the water concentration of theproduct stream, e.g., a water concentration of no more than 0.2 wt. %,e.g., no more than 0.15 wt. %, no more than 0.1 wt. %, or no more than0.05 wt. %.

Suitable cation exchange resins for the hydration of acetic anhydridemay comprise strong acid cation exchange resins, for example strong acidmacroreticular or macroporous resins, for example Amberlyst® 15 resin(DOW), Purolite C145, or Purolite CT145. In one embodiment, the cationexchange resin comprises sulfonic acid or partially metal impregnatedsulfonic acid functional groups. The resin may also be an acid-formstrong acid cation exchange mesoporous resin. Chelating resins andzeolites may also be used.

Advantageously, by hydrating the product stream in the cation exchangeresin, the acetic anhydride of the product stream may be reduced by atleast 60%, e.g., at least 70%, at least 75%, at least 80%, or at least85%. In one embodiment, the process for purifying acetic acid comprisesdistilling a process stream in a column, the process stream comprisingacetic acid and water, forming acetic anhydride in a lower portion ofthe column, withdrawing a product stream from the lower portion, whereinthe product stream comprises acetic anhydride, and hydrating the atleast 60% of the acetic anhydride in the product stream to form apurified acetic acid product.

As disclosed further herein, the product stream may contain iodide in aconcentration from 20 wppb to 1.5 wppm. The iodide may be metalion-exchange resin. The metal-exchanged ion exchange resin can have atleast 1% of the strong acid exchange sites occupied by silver, mercury,palladium, and/or rhodium, e.g., at least 2% silver, mercury, palladium,and/or rhodium, at least 5% silver, mercury, palladium, and/or rhodium,at least 10% silver, mercury, palladium, and/or rhodium, or at least 20%silver, mercury, palladium, and/or rhodium. The product stream have mayan iodide content of greater than 100 wppb, e.g., greater than 100 wppb,greater than 200 wppb, greater than 400 wppb, greater than 500 wppb, orgreater than 1000 wppb, prior to treatment with the metal ion-exchangeresin and an iodide content of less than 10 wppb, e.g., less than 10wppb, less than 7 wppb, less than 5 wppb, less than 3 wppb, less than 2wppb, less than 1 wppb, after contacting the resin.

Cation Removal

Accordingly in one embodiment, the cation exchange resin for hydratingacetic anhydride is position upstream of the metal ion-exchange resinfor removing iodides. This configuration may be desirable when, thecation exchange resin may also remove metal ion contaminants, inparticular lithium cations, that cause displacement of the metals of theion exchange resin. In one embodiment, the lithium cation in the productstream to be removed using the cation exchange resin may be derived fromand/or generated by a compound in the reaction medium. In someembodiments, the cation exchange resin may remove cations may beselected from the group consisting of Groups IA and IIA of the periodictable, quaternary nitrogen cations, and phosphorous-containing cations.Higher alkyl iodides, C₁₀-C₁₄ alkyl iodides, may also be removed usingthe cation exchange resin.

In one embodiment, the cation exchange resin may also reduce lithiumconcentration in the product stream. Lithium has also been found to beentrained in the crude acid product in the absence of heavy ends andother finishing apparatus. Even very small amounts of lithium in theproduct stream, e.g., 10 wppb, may cause problems with removing iodides.Lithium may be present in the product stream as one or morelithium-containing compounds such as lithium iodide, lithium hydroxide,lithium acetate, lithium acetate dihydrate, lithium carbonate, lithiumalkyl carbonate, methyl lithium, lithium chloride, or lithium oxalate.For purposes herein, the concentration of lithium-containing compound isreported as the concentration of the lithium in the lithium-containingcompound. The product stream may comprise lithium in a concentration ofup to or equal to 10 wppm, e.g., up to or equal to 5 wppm, up to orequal to 1 wppm, up to or equal to 500 wppb, up to or equal to 300 wppb,or up to or equal to 100 wppb. In terms of ranges, the crude acidproduct may comprise lithium in an amount from 0.01 wppm to 10 wppm,e.g., from 0.05 wppm to 5 wppm or from 0.05 wppm to 1 wppm. By utilizinga cationic exchanger in the acid form before introducing the crude acidproduct to a metal-exchanged resin, significant amounts of lithium canbe removed. For example greater than or equal to 90 wt. % of the lithiumin the stream may be removed by the cationic exchanger, e.g., greaterthan or equal to 92 wt. %, greater than or equal to 95 wt. %, greaterthan or equal to 98 wt. %, or greater than or equal to 99 wt. %. Thus,the stream exiting the acid-form cationic exchanger may contain no morethan 50 wppb lithium, e.g., less than 25 wppb lithium, no more than 10wppb, or no more than 5 wppb. Such removal of the lithium can greatlyextend the life of the metal-exchanged resin.

In other embodiments, the lithium concentration in the product streammay be controlled by removing the product stream as a vapor sidedrawfrom the distillation column. When a vapor sidedraw is used, the productstream is condensed prior to contacting the cation exchange resin tohydrate the acetic anhydride and, optionally, remove cations.

In other embodiments, the cation exchange resin may be positioneddownstream of the metal ion-exchange resin for removing iodides, orpositioned in parallel with the metal ion-exchange resin for treating aportion of the product stream.

Acetic Acid Production Systems

An exemplary acetic acid production process is described below. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. It will of course be appreciated thatin the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

In one embodiment, there is provided a process for producing acetic acidcomprising carbonylating, in a reactor, at least one member selectedfrom the group consisting of methanol, dimethyl ether, and methylacetate in a reaction medium comprising water at a concentration from0.1 to 14 wt. %, a rhodium catalyst, methyl iodide, and an iodide salt,separating the reaction medium to obtain a process stream comprisingacetic acid and water, distilling the process stream in a column toobtain a product stream comprising acetic acid, water at a concentrationof no more than 0.2 wt. % and acetic anhydride at a concentration of nomore than 600 wppm and contacting the product stream with a cationexchange resin to form a purified acetic acid product comprising no morethan 50 wppm acetic anhydride.

The purification processes described herein may be useful incarbonylation processes that use methanol and/or methyl acetate (MeAc),methyl formate or dimethyl ether, or mixtures thereof, to produce aceticacid in the presence of a Group VIII metal catalyst, such as rhodium,and a halogen-containing catalyst promoter. A particularly usefulprocess is the low water rhodium-catalyzed carbonylation of methanol toacetic acid as exemplified in U.S. Pat. No. 5,001,259. Other metalcatalysts, e.g., iridium-based catalysts, are contemplated as well.

Generally, the metal component, e.g., rhodium component, of the catalystsystem is believed to be present in the form of a coordination compoundof rhodium with a halogen component providing at least one of theligands of such coordination compound. In addition to the coordinationof rhodium and halogen, it is also believed that carbon monoxidecoordinates with rhodium. The rhodium component of the catalyst systemmay be provided by introducing into the reaction zone rhodium in theform of rhodium metal, rhodium salts such as the oxides, acetates,iodides, carbonates, hydroxides, chlorides, etc., or other compoundsthat result in the formation of a coordination compound of rhodium inthe reaction environment.

The metal catalyst may comprise a Group VIII metal. Suitable Group VIIIcatalysts include rhodium and/or iridium catalysts. When a rhodiumcatalyst is used, the rhodium catalyst may be added in any suitable formsuch that rhodium is in the catalyst solution as an equilibrium mixtureincluding [Rh(CO)₂I₂]-anion, as is well known in the art. Iodide saltsoptionally maintained in the reaction mixtures of the processesdescribed herein may be in the form of a soluble salt of an alkali metalor alkaline earth metal, quaternary ammonium, phosphonium salt ormixtures thereof. In certain embodiments, the catalyst co-promoter islithium iodide, lithium acetate, or mixtures thereof. The saltco-promoter may be added as a non-iodide salt that generates an iodidesalt. The iodide catalyst stabilizer may be introduced directly into thereaction system. Alternatively, the iodide salt may be generated in-situsince under the operating conditions of the reaction system, a widerange of non-iodide salt precursors reacts with methyl iodide orhydroiodic acid in the reaction medium to generate the correspondingco-promoter iodide salt stabilizer. For additional detail regardingrhodium catalysis and iodide salt generation, see U.S. Pat. Nos.5,001,259; 5,026,908; 5,144,068 and 7,005,541, the entireties of whichare hereby incorporated by reference. The carbonylation of methanolutilizing iridium catalyst is well known and is generally described inU.S. Pat. Nos. 5,942,460, 5,932,764, 5,883,295, 5,877,348, 5,877,347 and5,696,284, the entireties of which are hereby incorporated by reference.

The halogen-containing catalyst promoter of the catalyst system consistsof a halogen compound comprising an organic halide. Thus, alkyl, aryl,and substituted alkyl or aryl halides can be used. Preferably, thehalogen-containing catalyst promoter is present in the form of an alkylhalide. Even more preferably, the halogen-containing catalyst promoteris present in the form of an alkyl halide in which the alkyl radicalcorresponds to the alkyl radical of the feed alcohol, which is beingcarbonylated. Thus, in the carbonylation of methanol to acetic acid, thehalide promoter may include methyl halide, and more preferably methyliodide.

The components of the reaction medium are maintained within definedlimits to ensure sufficient production of acetic acid as the primaryproduct and thus are not directed to co-production process for producingacetic acid along with a secondary product, such as acetic anhydride, inthe reaction medium The reaction medium contains a concentration of themetal catalyst, e.g., rhodium catalyst, in an amount from 200 to 3000wppm as rhodium, e.g., from 500 to 2000 wppm, or from 600 to 1500 wppm.The concentration of water in the reaction medium is maintained underlow water conditions, e.g., water in amount of no more than 14 wt. %,from 0.1 wt. % to 14 wt. %, from 0.2 wt. % to 10 wt. % or mostpreferably from 0.25 wt. % to 5 wt. %. The concentration of methyliodide in the reaction medium is maintained to be from 1 to 25 wt. %,e.g., from 5 to 20 wt. %, from 4 to 13.9 wt. %. The concentration ofiodide salt, e.g., lithium iodide, in the reaction medium is maintainedto be from 1 to 25 wt. %, e.g., from 2 to 20 wt. %, from 3 to 20 wt. %.The concentration of methyl acetate in the reaction medium is maintainedto be from 0.5 to 30 wt. %, e.g., from 0.3 to 20 wt. %, from 0.6 to 4.1wt. %. The concentration of acetic acid in the reaction medium isgenerally greater than or equal to 30 wt. %, e.g., greater than or equalto 40 wt. %, greater than or equal to 50 wt. %, or greater than or equalto 60 wt. %. Similar to the process stream disclosed herein, thereaction medium is substantially free of acetic anhydride. The followingamounts are based on the total weight of the reaction medium.

In embodiments, the process for producing acetic acid further includesintroducing a lithium compound into the reactor to maintain theconcentration of lithium acetate in an amount from 0.3 to 0.7 wt. % inthe reaction medium, wherein in an exemplary embodiment, in the reactionmedium the concentration of the rhodium catalyst is maintained in anamount from 200 to 3000 wppm as rhodium in the reaction medium, theconcentration of water is maintained in amount from 0.1 to 4.1 wt. % inthe reaction medium, and the concentration of methyl acetate ismaintained from 0.6 to 4.1 wt. % in the reaction medium, based on thetotal weight of the reaction medium present within the carbonylationreactor.

In embodiments, the lithium compound introduced into the reactor isselected from the group consisting of lithium acetate, lithiumcarboxylates, lithium carbonates, lithium hydroxide, other organiclithium salts, and mixtures thereof. In embodiments, the lithiumcompound is soluble in the reaction medium. In an embodiment, lithiumacetate dihydrate may be used as the source of the lithium compound.

Lithium acetate reacts with hydrogen iodide according to the followingequilibrium reaction (I) to form lithium iodide and acetic acid:

LiOAc+HI⇄LiI+HOAc  (I)

Lithium acetate is thought to provide improved control of hydrogeniodide concentration relative to other acetates, such as methyl acetate,present in the reaction medium. Without being bound by theory, lithiumacetate is a conjugate base of acetic acid and thus reactive towardhydrogen iodide via an acid-base reaction. This property is thought toresult in an equilibrium of the reaction (I) which favors reactionproducts over and above that produced by the corresponding equilibriumof methyl acetate and hydrogen iodide. This improved equilibrium isfavored by water concentrations of less than 4.1 wt. % in the reactionmedium. In addition, the relatively low volatility of lithium acetatecompared to methyl acetate allows the lithium acetate to remain in thereaction medium except for volatility losses and small amounts ofentrainment into the vapor crude product. In contrast, the relativelyhigh volatility of methyl acetate allows the material to distill intothe purification train, rendering methyl acetate more difficult tocontrol. Lithium acetate is much easier to maintain and control in theprocess at consistent low concentrations of hydrogen iodide.Accordingly, a relatively small amount of lithium acetate may beemployed relative to the amount of methyl acetate needed to controlhydrogen iodide concentrations in the reaction medium. It has furtherbeen discovered that lithium acetate is at least three times moreeffective than methyl acetate in promoting methyl iodide oxidativeaddition to the rhodium [I] complex. However, it has been discoveredthat lithium cations derived from and/or generated by the lithiumcompound in the reaction medium may be entrained or be volatile enoughto concentrate with the crude acetic acid product after purification inthe primary purification trains.

In embodiments, the concentration of lithium acetate in the reactionmedium is maintained at greater than or equal to 0.3 wt. %, or greaterthan or equal to 0.35 wt. %, or greater than or equal to 0.4 wt. %, orgreater than or equal to 0.45 wt. %, or greater than or equal to 0.5 wt.%, and/or in embodiments, the concentration of lithium acetate in thereaction medium is maintained at no more than 0.7 wt. %, or no more than0.65 wt. %, or no more than 0.6 wt. %, or no more than 0.55 wt. %.

It has been discovered that an excess of lithium acetate in the reactionmedium can adversely affect the other compounds in the reaction medium,leading to decrease productivity. Conversely, it has been discoveredthat a lithium acetate concentration in the reaction medium below about0.3 wt. % is unable to maintain the desired hydrogen iodideconcentrations in the reaction medium of below 1.3 wt. %.

In embodiments, the lithium compound may be introduced continuously orintermittently into the reaction medium. In embodiments, the lithiumcompound is introduced during reactor start up. In embodiments, thelithium compound is introduced intermittently to replace entrainmentlosses.

Thus, in one embodiment, there is provided a process for producingacetic acid comprising carbonylating, in a reactor, at least one memberselected from the group consisting of methanol, dimethyl ether, andmethyl acetate in a reaction medium comprising water at a concentrationfrom 0.1 to 14 wt. %, a rhodium catalyst, methyl iodide, an iodide salt,and lithium acetate, separating the reaction medium to obtain a processstream comprising acetic acid and water, distilling the process streamin a column to obtain a product stream comprising acetic acid, water ata concentration of no more than 0.2 wt. % and acetic anhydride at aconcentration of no more than 600 wppm and contacting the product streamwith a cation exchange resin to form a purified acetic acid productcomprising no more than 50 wppm acetic anhydride.

In some embodiments, the desired reaction rates are obtained even at lowwater concentrations by maintaining in the reaction medium an ester ofthe desired carboxylic acid and an alcohol, desirably the alcohol usedin the carbonylation, and an additional iodide ion that is over andabove the iodide ion that is present as hydrogen iodide. A desired esteris methyl acetate. The additional iodide ion is desirably an iodidesalt, with lithium iodide (LiI) being preferred. It has been found, asdescribed in U.S. Pat. No. 5,001,259, that under low waterconcentrations, methyl acetate and lithium iodide act as rate promoters.

The carbonylation reaction of methanol to acetic acid product may becarried out by contacting the methanol feed with gaseous carbon monoxidebubbled through an acetic acid solvent reaction medium containing therhodium catalyst, methyl iodide promoter, methyl acetate, and additionalsoluble iodide salt, at conditions of temperature and pressure suitableto form the carbonylation product. It will be generally recognized thatit is the concentration of iodide ion in the catalyst system that isimportant and not the cation associated with the iodide, and that at agiven molar concentration of iodide the nature of the cation is not assignificant as the effect of the iodide concentration. Any metal iodidesalt, or any iodide salt of any organic cation, or other cations such asthose based on amine or phosphine compounds (optionally, ternary orquaternary cations), can be maintained in the reaction medium providedthat the salt is sufficiently soluble in the reaction medium to providethe desired level of the iodide. When the iodide is a metal salt,preferably it is an iodide salt of a member of the group consisting ofthe metals of Group IA and Group IIA of the periodic table as set forthin the “Handbook of Chemistry and Physics” published by CRC Press,Cleveland, Ohio, 2002-03 (83rd edition). In particular, alkali metaliodides are useful, with lithium iodide being particularly suitable. Inthe low water carbonylation process, the additional iodide ion over andabove the iodide ion present as hydrogen iodide is generally present inthe catalyst solution in amounts such that the total iodide ionconcentration is from 1 to 25 wt. % and the methyl acetate is generallypresent in amounts from 0.5 to 30 wt. %, and the methyl iodide isgenerally present in amounts from 1 to 25 wt. %. The rhodium catalyst isgenerally present in amounts from 200 to 3000 wppm as rhodium.

The reaction medium may also contain impurities that should becontrolled to avoid byproduct formation. One impurity in the reactionmedium may be ethyl iodide, which is difficult to separate from aceticacid. Applicant has further discovered that the formation of ethyliodide may be affected by numerous variables, including theconcentration of acetaldehyde, ethyl acetate, methyl acetate and methyliodide in the reaction medium. Additionally, ethanol content in themethanol source, hydrogen partial pressure and hydrogen content in thecarbon monoxide source have been discovered to affect ethyl iodideconcentration in the reaction medium and, consequently, propionic acidconcentration in the final acetic acid product.

In embodiments, the propionic acid concentration in the acetic acidproduct may further be maintained below 250 wppm by maintaining theethyl iodide concentration in the reaction medium at no more than 750wppm without removing propionic acid from the acetic acid product.

In embodiments, the ethyl iodide concentration in the reaction mediumand propionic acid in the acetic acid product may be present in a weightratio from 3:1 to 1:2. In embodiments, the acetaldehyde:ethyl iodideconcentration in the reaction medium is maintained at a weight ratiofrom 2:1 to 20:1.

In embodiments, the ethyl iodide concentration in the reaction mediummay be maintained by controlling at least one of the hydrogen partialpressure, the methyl acetate concentration, the methyl iodideconcentration, and/or the acetaldehyde concentration in the reactionmedium.

In embodiments, the concentration of ethyl iodide in the reaction mediumis maintained/controlled to be no more than 750 wppm, or e.g., no morethan 650 wppm, or no more than 550 wppm, or no more than 450 wppm, or nomore than 350 wppm. In embodiments, the concentration of ethyl iodide inthe reaction medium is maintained/controlled at greater than or equal to1 wppm, or e.g., 5 wppm, or 10 wppm, or 20 wppm, or 25 wppm, and no morethan 650 wppm, or e.g., 550 wppm, or 450 wppm, or 350 wppm.

In embodiments, the weight ratio of ethyl iodide in the reaction mediumto propionic acid in the acetic acid product may range from 3:1 to 1:2,or e.g., from 5:2 to 1:2, or from 2:1 to 1:2, or from 3:2 to 1:2.

In embodiments, the weight ratio of acetaldehyde to ethyl iodide in thereaction medium may range from 20:1 to 2:1, or e.g., from 15:1 to 2:1,from 9:1 to 2:1, or from 6:1.

Carbonylation Reaction

Typical reaction temperatures for carbonylation may be from 150 to 250°C., e.g., 160 to 240° C., 170-230° C. with the temperature range of 180to 225° C. being a preferred range. The carbon monoxide partial pressurein the reactor can vary widely but is typically from 2 to 30 atm, e.g.,from 3 to 10 atm. The hydrogen partial pressure in the reactor istypically from 0.05 to 2 atm, e.g., from 1 to 1.9 atm. Some embodimentsof the present invention may be operated with a hydrogen partialpressure from 0.3 to 2 atm, e.g., from 0.3 to 1.5 atm, or from 0.4 to1.5 atm. Because of the partial pressure of by-products and the vaporpressure of the contained liquids, the total reactor pressure may rangefrom 15 to 40 atm. The production rate of acetic acid may be from 5 to50 mol/L·h, e.g., from 10 to 40 mol/L·h, and preferably 15 to 35mol/L·H. As indicated herein, the carbonylation reaction is conductedunder conditions that do not produce acetic anhydride.

Exemplary reaction and acetic acid recovery system 100 is shown inFIG. 1. As shown, methanol-containing feed stream 101 and carbonmonoxide-containing feed stream 102 are directed to liquid phasecarbonylation reactor 104, in which the carbonylation reaction occurs toform acetic acid.

Carbonylation reactor 104 is preferably either a stirred vessel orbubble-column type vessel, with or without an agitator, within which thereacting liquid or slurry contents are maintained, preferablyautomatically, a predetermined level, which preferably remainssubstantially constant during normal operation. Into carbonylationreactor 104, fresh methanol, carbon monoxide, and sufficient water arecontinuously introduced as needed to maintain suitable concentrations inthe reaction medium.

In a typical carbonylation process, carbon monoxide is continuouslyintroduced into the carbonylation reactor, desirably below the agitator,which may be used to stir the contents. The gaseous feed preferably isthoroughly dispersed through the reacting liquid by this stirring means.Gaseous purge stream 106 desirably is vented from the reactor 104 toprevent buildup of gaseous by-products and to maintain a set carbonmonoxide partial pressure at a given total reactor pressure. In oneembodiment, the gaseous purge stream 106 contains low amounts ofhydrogen iodide of no more than 1 wt. %, e.g., no more than 0.9 wt. %,no more than 0.8 wt. %, no more than 0.7 wt. %, no more than 0.5 wt. %,no more than 0.3 wt. %. Hydrogen iodide in excess of these amounts mayincrease the duty on the scrubber to prevent hydrogen iodide from beingpurged. The temperature of the reactor may be controlled and the carbonmonoxide feed is introduced at a rate sufficient to maintain the desiredtotal reactor pressure. Stream 113 comprising the liquid reaction mediumexits reactor 104.

The acetic acid production system preferably includes primarypurification train 108 employed to recover the acetic acid and recyclecatalyst solution, methyl iodide, methyl acetate, and other systemcomponents within the process. Primary purification train 108 includelight ends column 124 and drying column 130, and the associated pumps,overhead receivers, condensers, etc. Thus, a recycled catalyst solution,such as stream 110 from flash vessel 112, and optionally one or more ofrecycle streams 114, 116, 118, and 120, also are introduced into thereactor 104. Of course, one or more of the recycle streams may becombined prior to being introduced into the reactor. The separationsystem also preferably controls water and acetic acid content in thecarbonylation reactor, as well as throughout the system, and facilitatesPRC removal.

Flash Vessel

The reaction medium is drawn off from the carbonylation reactor 104 at arate sufficient to maintain a constant level therein and is provided toflash vessel 112 via stream 113. In flash vessel 112, the crude productis separated in a flash separation step to obtain a vapor product stream122 comprising acetic acid and less volatile stream 110, e.g., a liquidrecycle stream, comprising a catalyst-containing solution (predominantlyacetic acid containing the rhodium and the iodide salt along with lesserquantities of methyl acetate, methyl iodide, and water), whichpreferably is recycled to the reactor, as discussed above. The vaporproduct stream 122 also comprises methyl iodide, methyl acetate, water,and permanganate reducing compounds (PRC's). Dissolved gases exiting thereactor and entering the flash vessel comprise a portion of the carbonmonoxide and may also contain gaseous by-products such as methane,hydrogen, and carbon dioxide. Such dissolved gases exit the flash vesselas part of the overhead stream.

In one embodiment, vapor product stream 122 comprises acetic acid,methyl iodide, methyl acetate, water, acetaldehyde, and hydrogen iodide.In one embodiment, vapor product stream 122 comprises acetic acid in anamount from 45 to 75 wt. %, methyl iodide in an amount from 20 to 50 wt.%, methyl acetate in an amount of no more than 9 wt. %, and water in anamount of no more than 15 wt. %, based on the total weight of the vaporproduct stream. In another embodiment, vapor product stream 122comprises acetic acid in an amount from 45 to 75 wt. %, methyl iodide inan amount from 24 to less than 36 wt. %, methyl acetate in an amount ofno more than 9 wt. %, and water in an amount of no more than 15 wt. %,based on the total weight of the vapor product stream. More preferably,vapor product stream 122 comprises acetic acid in an amount from 55 to75 wt. %, methyl iodide in an amount from 24 to 35 wt. %, methyl acetatein an amount from 0.5 to 8 wt. %, and water in an amount from 0.5 to 14wt. %. In yet a further preferred embodiment, vapor product stream 112comprises acetic acid in an amount from 60 to 70 wt. %, methyl iodide inan amount from 25 to 35 wt. %, methyl acetate in an amount from 0.5 to6.5 wt. %, and water in an amount from 1 to 8 wt. %. The acetaldehydeconcentration in the vapor product stream may be in an amount from 0.005to 1 wt. %, based on the total weight of the vapor product stream, e.g.,from 0.01 to 0.8 wt. %, or from 0.01 to 0.7 wt. %. In some embodimentsthe acetaldehyde may be present in amounts no more than 0.01 wt. %.Vapor product stream 122 may comprise hydrogen iodide in an amount nomore than 1 wt. %, based on the total weight of the vapor productstream, e.g., no more than 0.5 wt. %, or no more than 0.1 wt. %. Vaporproduct stream 122 is preferably substantially free of, i.e., containsno more than 0.0001 wt. %, propionic acid, based on the total weight ofthe vapor product stream. Similar to the process stream disclosedherein, vapor product stream 122 is substantially free of aceticanhydride.

Less volatile stream 110 comprises acetic acid, the metal catalyst,corrosion metals, as well as other various compounds. In one embodiment,liquid recycle stream comprises acetic acid in an amount from 60 to 90wt. %, metal catalyst in an amount from 0.01 to 0.5 wt. %; corrosionmetals (e.g., nickel, iron and chromium) in a total amount from 10 to2500 wppm; lithium iodide in an amount from 5 to 20 wt. %; methyl iodidein an amount from 0.5 to 5 wt. %; methyl acetate in an amount from 0.1to 5 wt. %; water in an amount from 0.1 to 8 wt. %; acetaldehyde in anamount of no more than 1 wt. % (e.g., from 0.0001 to 1 wt. %acetaldehyde); and hydrogen iodide in an amount of no more than 0.5 wt.% (e.g., from 0.0001 to 0.5 wt. % hydrogen iodide).

Recovery of Acetic Acid

The distillation and recovery of acetic acid is not particularly limitedfor the purposes of the present invention. In one embodiment, theprocess involves carbonylating, in a reactor, at least one memberselected from the group consisting of methanol, dimethyl ether, andmethyl acetate in a reaction medium comprising water at a concentrationfrom 0.1 to 14 wt. %, a rhodium catalyst, methyl iodide, and an iodidesalt, separating the reaction medium to form a liquid recycle stream anda vapor product stream, distilling at least a portion of the vaporproduct stream in a first column to obtain a side stream comprisingacetic acid at a concentration greater than 90 wt. %, water at aconcentration from 1 to 3 wt. %, one or more C₁-C₁₄ alkyl iodides in atotal concentration of no more than 6 wt. % and methyl acetate at aconcentration of no more than 6 wt. %, distilling the side stream in asecond column to obtain a product stream comprising acetic acid, waterat a concentration of no more than 0.2 wt. % and acetic anhydride at aconcentration of no more than 600 wppm, and contacting the productstream with a cation exchange resin to form a purified acetic acidproduct comprising no more than 50 wppm acetic anhydride. Variousembodiments of primary purification train having up to 2 distillationcolumns is further described herein.

First Column

For purposes of the present invention a process stream refers to anystream that is fed to the distillation column. In one embodiment vaporproduct stream 122 may be a process stream. The overhead stream fromflash vessel 112 is directed to the light ends column 124 (first column)as vapor product stream 122, where distillation yields a low-boilingoverhead vapor stream 126, a sidedraw 128 that contains acetic acid, anda high boiling residue stream 116. In one embodiment, vapor productstream 122 may comprise acetic acid, methyl acetate, water, methyliodide, and acetaldehyde, along with other impurities such as hydrogeniodide and crotonaldehyde, and byproducts such as propionic acid. Aceticacid removed via sidedraw 128 preferably is subjected to furtherpurification, such as in drying column 130 (second column) for selectiveseparation of acetic acid from water as described further herein.

In one embodiment, sidedraw 128 is removed from a location above thefeed of vapor product stream 122 to light ends column 124. Thus, aceticanhydride does not form because the water concentrations in thatlocation of light ends column 124 at greater than 0.2 wt. %, greaterthan 0.5 wt. %, or greater than 1 wt. %.

Light ends column 124 also preferably forms residuum or bottoms stream116, which comprises primarily acetic acid and water. Since light endsbottoms stream 116 typically comprises some residual catalyst, it may bebeneficial to recycle all or a portion of light ends bottoms stream 116to reactor 104. Optionally, light ends bottoms stream 116 may becombined with the catalyst phase 110 from flash vessel 112 and returnedtogether to reactor 104, as shown in FIG. 1. Although the concentrationof acetic acid may be relatively high in high boiling residue stream116, the mass flow of the high boiling residue stream 116 relative toside stream 128 is very small. In embodiments, the mass flow of theboiling residue stream 116 is no more than 0.75% of side stream 128,e.g., no more than 0.55%, or no more than 0.45%.

In one embodiment, low-boiling overhead vapor stream 126 comprises waterin amount greater than or equal to 5 wt. %, e.g., greater than or equalto 10 wt. %, or greater than or equal to 25 wt. %. The amount of watermay be up to 80 wt. %. In terms of ranges, water concentration in theoverhead may be from 5 wt. % to 80 wt. %, e.g., from 10 wt. % to 70 wt.% or from 25 wt. % to 60 wt. %. Reducing water concentration to lessthan 5 wt. % is not advantageous because this results in a large recycleof acetic acid back to the reaction system which then sets up a largerecycle through the entire purification system. In addition to water,low-boiling overhead vapor stream 126 may also comprise methyl acetate,methyl iodide, and carbonyl impurities, which are preferablyconcentrated in the overhead to be removed from acetic acid in sidestream 128. These carbonyl impurities may also be referred to herein asPRC's.

As shown, low-boiling overhead vapor stream 126 preferably is condensedand directed to an overhead phase separation unit, as shown by overheaddecanter 134. Conditions are desirably maintained such that thecondensed low-boiling overhead vapor stream 126, once in decanter 134,may separate to form a light liquid phase 138 and a heavy liquid phase118. The phase separation should be maintain two separate phase, withoutforming a third phase or emulsion between the phases. An offgascomponent may be vented via line 136 from decanter 134. In embodiments,the average residence time of the condensed low-boiling overhead vaporstream 126 in overhead decanter 134 is greater than or equal to 1minute, e.g., greater than or equal to 3 minutes, greater than or equalto 5 minutes, greater than or equal to 10 minutes, and/or the averageresidence time is no more than 60 minutes, e.g., no more than 45minutes, or no more than 30 minutes, or no more than 25 minutes.

Although the specific compositions of the light phase stream 138 mayvary widely, some preferred compositions are provided below in Table 1.

TABLE 1 Exemplary Light Liquid Phase from Light Ends Overhead conc. (Wt.%) conc. (Wt. %) conc. (Wt. %) HOAc 1-40 1-25 5-15 Water 50-90  50-80 60-80  PRC's  <5 <3 <1 MeI <10 <5 <3 MeAc 1-50 1-25 1-15

In one embodiment, overhead decanter 134 is arranged and constructed tomaintain a low interface level to prevent an excess hold up of methyliodide. Although the specific compositions of heavy liquid phase 118 mayvary widely, some exemplary compositions are provided below in Table 2.

TABLE 2 Exemplary Heavy Liquid Phase from Light Ends Overhead conc. (Wt.%) conc. (Wt. %) conc. (Wt. %) Water 0.01-2  0.05-1   0.1-0.9 MethylAcetate 0.1-25 0.5-20  0.7-15  Acetic Acid 0.1-10 0.2-8   0.5-6   PRC's<5 <3 <1 Methyl Iodide  40-98 50-95 60-85

The density of the heavy liquid phase 118 may be from 1.3 to 2, e.g.,from 1.5 to 1.8, from 1.5 to 1.75 or from 1.55 to 1.7. As described inU.S. Pat. No. 6,677,480, the measured density in the heavy liquid phase118 correlates with the methyl acetate concentration in the reactionmedium. As density decreases, the methyl acetate concentration in thereaction medium increases. In one embodiment of the present inventionheavy liquid phase 118 is recycled to the reactor and the light liquidphase 138 is controlled to be recycled through the same pump. It may bedesirable to recycle a portion of the light liquid phase 138 that doesnot disrupt the pump and maintains a density of the combined lightliquid phase 138 and heavy liquid phase of greater than or equal to 1.3,e.g., greater than or equal to 1.4, greater than or equal to 1.5, orgreater than or equal to 1.7. As described herein, a portion of theheavy liquid phase 118 may be treated to remove impurities such asacetaldehyde.

As shown in FIG. 1, the light phase exits decanter 134 via stream 138. Afirst portion, e.g., aliquot portion, of light phase stream 138 isrecycled to the top of the light ends column 124 as reflux stream 140.In other embodiments a portion of the heavy liquid phase 118 may also berefluxed (not shown) to the light ends column 124.

PRC Removal System

As described herein the light ends column 124 is part of the primarypurification train. In some embodiments, a portion of light liquid phaseand/or heavy liquid phase may be separated and directed to acetaldehydeor PRC removal system 132 to recover methyl iodide and methyl acetate,while removing acetaldehyde. For purposes of the present invention, theacetaldehyde or PRC removal system 132 is not part of the primarypurification train.

As shown in Tables 1 and 2, light liquid phase 133 and/or heavy liquidphase 118 each contain PRC's and the process may include removingcarbonyl impurities, such as acetaldehyde, that deteriorate the qualityof the acetic acid product and may be removed in suitable impurityremoval columns and absorbers as described in U.S. Pat. Nos. 6,143,930;6,339,171; 7,223,883; 7,223,886; 7,855,306; 7,884,237; 8,889,904; and USPub. Nos. 2006/0011462, which are incorporated herein by reference intheir entirety. Carbonyl impurities, such as acetaldehyde, may reactwith iodide catalyst promoters to form alkyl iodides, e.g., ethyliodide, propyl iodide, butyl iodide, pentyl iodide, hexyl iodide, etc.Also, because many impurities originate with acetaldehyde, it isdesirable to remove carbonyl impurities from the liquid light phase.

The portion of light liquid phase 138 and/or heavy liquid phase 118 fedto the acetaldehyde or PRC removal system 132 via lines 142 and 143,respectively, may vary from 1% to 99% of the mass flow of either thelight liquid phase 138 and/or heavy liquid phase 118, e.g., from 1 to50%, from 2 to 45%, from 5 to 40%, 5 to 30% or 5 to 20%. Also in someembodiments, a portion of both the light liquid phase 138 and heavyliquid phase 118 may be fed to the acetaldehyde or PRC removal system132. The portion of the light liquid phase 138 not fed to theacetaldehyde or PRC removal system 132 may be refluxed to the firstcolumn or recycled to the reactor, as described herein. The portion ofthe heavy liquid phase 118 not fed to the acetaldehyde or PRC removalsystem 132 may be recycled to the reactor. Although a portion of heavyliquid phase 118 may be refluxed to the light ends column, it is moredesirable to return the methyl iodide enriched heavy liquid phase 118 tothe reactor.

In one embodiment, a portion of light liquid phase 138 and/or heavyliquid phase 118 is fed to a distillation column which enriches theoverhead thereof to have acetaldehyde and methyl iodide. Depending onthe configuration, there may be two separate distillation columns, andthe overhead of the second column may be enriched in acetaldehyde andmethyl iodide. Dimethyl ether, which may be formed in-situ, may also bepresent in the overhead. The overhead may be subject to one or moreextraction stages to remove a raffinate enriched in methyl iodide and anextractant. A portion of the raffinate may be returned to thedistillation column, first column, overhead decanter and/or reactor. Forexample, when the heavy liquid phase 118 is treated in the PRC removalsystem 132, it may be desirable to return a portion the raffinate toeither the distillation column or reactor. Also, for example, when lightliquid phase 138 is treated in the PRC removal system 132, it may bedesirable to return a portion the raffinate to either the first column,overhead decanter, or reactor. In some embodiments, the extractant maybe further distilled to remove water, which is returned to the one ormore extraction stages. The column bottoms, which contains more methylacetate and methyl iodide than light liquid phase 138, may also berecycled to reactor 104 and/or refluxed to light ends column 124.

Drying Column

Returning to the primary purification train, in addition to the overheadphase, the light ends column 124 also forms an acetic acid sidedraw 128,which preferably comprises primarily acetic acid and water, and issubstantially free of acetic anhydride. In one embodiment, acetic acidsidedraw 128 is a process stream. In one embodiment, acetic acidsidedraw 128 comprises acetic acid in amount of greater than or equal to90 wt. %, e.g., greater than or equal to 94 wt. % or greater than orequal to 96 wt. %. The water concentration of the process stream may bein an amount from 1 to 3 wt. %, e.g., from 1 to 2.5 wt. % and morepreferably from 1.1 to 2.1 wt. %. The acetic acid sidedraw 128 may alsocomprise one or more C₁-C₁₄ alkyl iodides in a total concentration of nomore than 6 wt. %, e.g., no more than 4 wt. %, or no more than 3.6 wt.%, and methyl acetate at a concentration of no more than 6 wt. %, e.g.,no more than 4 wt. %, or no more than 3.6 wt. %. In some embodiments, inaddition to acetic acid and water, acetic acid sidedraw 128 may alsocomprise one or more C₁-C₁₄ alkyl iodides in an amount from 0.1 to 6 wt.%, e.g., from 0.5 to 5 wt. %, from 0.6 to 4 wt. %, from 0.7 to 3.7 wt.%, or from 0.8 to 3.6 wt. %. Due to the presence of water, acetic acidsidedraw 128 may also contain methyl acetate in an amount from 0.1 to 6wt. %, e.g., from 0.5 to 5 wt. %, from 0.6 to 4 wt. %, from 0.7 to 3.7wt. %, or from 0.8 to 3.6 wt. %. In some embodiments, acetic acidsidedraw 128 may also comprise hydrogen iodide at a concentration of nomore than 300 wppm, e.g., or no more than 250 wppm, no more than 200wppm, no more than 100 wppm, no more than 50 wppm, no more than 25 wppm,or no more than 10 wppm.

In one embodiment, to maintain an efficient product separation, it ishighly desired that the composition of the sidedraw 128 does not vary orfluctuate significantly during normal operation. By does not vary orfluctuate significantly it is meant that the concentration of the one ormore C₁-C₁₄ alkyl iodides and the concentration of methyl acetate is±0.9% of the water concentration in the side stream, e.g., ±0.7%, ±0.5%,±0.4%, ±0.3%, ±0.2%, or ±0.1%. The water concentration in the sidestream may be from 1 to 3 wt. %, e.g., preferably from 1.1 to 2.5 wt. %.For example, when the water concentration is 2.5 wt. %, theconcentration of C₁-C₁₄ alkyl iodides is from 1.6 to 3.4 wt. %, and theconcentration of methyl acetate is from 1.6 to 3.4 wt. %.

Optionally, a portion of the sidedraw 128 may be recirculated to thelight ends column, preferably to a point below where sidedraw 128 wasremoved from light ends column, in order to improve the separation (notshown).

Since sidedraw 128 contains water in addition to acetic acid, sidedraw128 from the light ends column 124 preferably is directed to dryingcolumn 130, in which the acetic acid and water are separated from oneanother. As shown, drying column 130, separates acetic acid sidedraw 128to form overhead stream 144 comprised primarily of water and a productstream 146 comprised primarily of acetic acid. Overhead stream 144preferably is cooled and condensed in a phase separation unit, e.g.,decanter 148, to form a light phase 150 and a heavy phase 122. As shown,a portion of the light phase is refluxed, as shown by stream 152 and theremainder of the light phase is returned to the reactor 104, as shown bystream 120. The heavy phase, which typically is an emulsion comprisingwater and methyl iodide, preferably is returned in its entirety to thereactor 104, as shown by stream 122, optionally after being combinedwith stream 120.

Exemplary compositions for the light phase of the drying column overheadare provided below in Table 3.

TABLE 3 Exemplary Light Phase Compositions from Drying Column Overheadconc. (Wt. %) conc. (Wt. %) conc. (Wt. %) HOAc 1-20 1-15 1-10 Water50-90  60-90  70-90  MeI <10 <5 <3 MeAc 1-20 1-15 1-10

In certain embodiments, as discussed, minor amounts of an alkalicomponent such as KOH can be added to sidedraw 128 via line 160 prior toentering the drying column 130. In other embodiments, the alkalicomponent might also be added to the drying column 130 at the sameheight level as the stream 128 entering the drying column 130 or at aheight above the height level height level as the stream 128 enteringthe drying column 130. Such addition can neutralize HI in the column.

Product stream 146 preferably comprises or consists essentially ofacetic acid. In further embodiments, it is preferred not to diluteproduct stream 146 with an aqueous diluent, such as water. In oneembodiment, product stream 146 comprises acetic acid at a concentrationof greater than or equal to 99.5 wt. %, e.g., greater than or equal to99.7 wt. % or greater than or equal to 99.9 wt. %. Product stream 146comprises water at a concentration of no more than 0.2 wt. %, e.g., nomore than 0.15 wt. %, no more than 0.1 wt. %, or no more than 0.05 wt.%. Due to anhydrous conditions in drying column 130, acetic anhydridemay be formed. In one embodiment, product stream 146 comprises aceticanhydride at a concentration of no more than 600 wppm, e.g., no morethan 500 wppm, no more than 450 wppm, no more than 400 wppm, no morethan 300 wppm, no more than 200 wppm, no more than 100 wppm, or no morethan 50 wppm. In terms of ranges, product stream 146 comprises aceticanhydride in an amount from 5 to 600 wppm, e.g., from 5 to 500 wppm,from 5 to 450 wppm, from 10 to 450 wppm, from 10 to 300 wppm, or from 10to 100 wppm.

In some embodiments, in addition to acetic anhydride, product stream 146may also comprise lithium in an amount of up to or equal to 10 wppm,e.g., up to or equal to 5 wppm, up to or equal to 1 wppm, up to or equalto 500 wppb, up to or equal to 300 wppb, or up to or equal to 100 wppb.

Thus, in one embodiment, there is provided a process for producingacetic acid comprising separating a reaction medium formed in a reactorin a flash vessel to form a liquid recycle and a vapor product stream,distilling the vapor product stream in a first column to obtain a sidestream and a low boiling overhead vapor stream comprising water in anamount of greater than or equal to 5 wt. %, condensing the low boilingoverhead vapor stream and biphasically separating the condensed streamto form a heavy liquid phase and a light liquid phase, optionallytreating a portion of the heavy liquid phase and/or the light liquidphase to remove at least one PRC, distilling the side stream in a secondcolumn to obtain a product stream comprising acetic acid, water at aconcentration of no more than 0.2 wt. % and acetic anhydride at aconcentration of no more than 600 wppm, and contacting the productstream with a cation exchange resin to form a purified acetic acidproduct comprising no more than 50 wppm acetic anhydride.

As shown in FIG. 2, in certain embodiments, the product stream withdrawnfrom the drying column 130 may be taken from a sidedraw 170 at aposition slightly above the bottom 172 of the column 130. In oneembodiment, sidedraw 170 is withdrawn within 5 actual stages from thebottom 172 of the column 130, e.g., within 4 actual stages from thebottom of the column 130, within 3 actual stages from the bottom of thecolumn 130, or within 2 actual stages from the bottom of the column 130.In some embodiments, the sidedraw 170 is withdrawn a position between 2and 5 actual stages from the bottom 172 of the column 130, e.g., aposition between 3 and 5 trays from the bottom of the column 130, orposition between 3 and 4 trays from the bottom of the column 130. In oneembodiment, the product stream may be in sidedraw 170 that is withdrawnin the liquid phase so that the lithium cation concentration would besimilar to the withdrawn lithium cation concentration in the dryingcolumns bottoms in stream 146. In other embodiments, sidedraw 170 may bea vapor stream and the lithium cation concentration may be less than thebottoms stream 146. When in the vapor phase, it is desirable to condensesidedraw 170 prior to contacting the cationic exchanger resin. When asidedraw 170 is used then other impurities such as heavy carbonylcontaining groups, i.e. propionic acid, may advantageously concentratein the bottoms stream 174. Residue stream 174 may be discarded or purgedfrom the process 100.

Sidedraw 170 contains the product stream that is contact with thecationic exchange resin to hydrate acetic anhydride according toembodiments of the present invention. In one embodiment there isprovided a process for producing acetic acid comprising separating areaction medium formed in a reactor in a flash vessel to form a liquidrecycle and a vapor product stream, distilling the vapor product streamin a first column to obtain a side stream and a low boiling overheadvapor stream comprising water in an amount of greater than or equal to 5wt. %, condensing the low boiling overhead vapor stream and biphasicallyseparating the condensed stream to form a heavy liquid phase and a lightliquid phase, optionally treating a portion of the heavy liquid phaseand/or the light liquid phase to remove at least one PRC, distilling theside stream in a second column to obtain a sidedraw, either in theliquid or vapor phase, wherein the sidedraw comprises acetic acid, waterat a concentration of no more than 0.2 wt. % and acetic anhydride at aconcentration of no more than 600 wppm, and contacting the sidedraw, ora condensed portion thereof, with a cation exchange resin to form apurified acetic acid product comprising no more than 50 wppm aceticanhydride.

In some embodiments, the product stream withdrawn from the bottoms orthe sidedraw may also be processed to remove lithium derived from and/orgenerated by the lithium compound in the reaction medium, by passingthrough cationic exchanger in the acid form and then through metalfunctionalized iodide removal ion exchange resins, prior to being storedor transported for commercial use. As described herein, cationicexchanger in the acid form are suitable for removing cations, such aslithium derived from and/or generated by compounds in the reactionmedium that concentrate in the crude acid product. Once thesecomponents, and in particular lithium, are removed the iodides may beremoved by metal functionalized iodide removal ion exchange resins.

Iodide Removal Beds/Use of Ion Exchange Resins

According to the present process, product stream that are contaminatedwith acetic anhydride and halides (e.g., iodides) may be contacted withan acid-form cationic exchange resin to hydrate the acetic anhydridefollowed by a metal-exchanged ion exchange resin having acid cationexchange sites comprising at least one metal selected from the groupconsisting of silver, mercury, palladium and rhodium under a range ofoperating conditions. Preferably, the ion exchange resin compositionsare provided in fixed beds. The use of fixed iodide removal beds topurify contaminated carboxylic acid streams is well documented in theart (see, for example, U.S. Pat. Nos. 4,615,806; 5,653,853; 5,731,252;and 6,225,498, which are hereby incorporated by reference in theirentireties). Generally, a contaminated liquid carboxylic acid stream iscontacted with the aforementioned ion exchange resin compositions, byflowing through a series of static fixed beds. In one embodiment, thecationic exchange resin may be used to hydrate at least 60% of theacetic anhydride in the product stream to yield a purified acetic acidproduct having acetic anhydride at a concentration of no more than 50wppm. In some embodiments, cations, such as lithium contaminants mayalso be removed by the cationic exchange resin in the acid form. Thehalide contaminants, e.g., iodide contaminants, are then removed byreaction with the metal of the metal-exchanged ion exchange resin toform metal iodides. In some embodiments, hydrocarbon moieties, e.g.,methyl groups, that may be associated with the iodide may esterify thecarboxylic acid. For example, in the case of acetic acid contaminatedwith methyl iodide, methyl acetate would be produced as a byproduct ofthe iodide removal. The formation of this esterification producttypically does not have a deleterious effect on the treated carboxylicacid stream.

Similar iodide contamination issues may exist in acetic anhydridemanufactured via a rhodium-iodide catalyst system. Thus, the inventiveprocess may alternatively be utilized in the purification of crudeacetic anhydride product streams.

Suitably stable ion exchange resins utilized in connection with thepresent invention for preparing silver or mercury-exchanged resins foriodide removal typically are of the “RSO₃H” type classified as “strongacid,” that is, sulfonic acid, cation exchange resins of themacroreticular (macroporous) type. Particularly suitable ion exchangesubstrates include Amberlyst® 15, Amberlyst® 35 and Amberlyst® 36 resins(DOW) suitable for use at elevated temperatures. Other stable ionexchange substrates such as zeolites may be employed, provided that thematerial is stable in the organic medium at the conditions of interest,that is, will not chemically decompose or release silver or mercury intothe organic medium in unacceptable amounts. Zeolite cationic exchangesubstrates are disclosed, for example, in U.S. Pat. No. 5,962,735, thedisclosure of which is incorporated herein by reference.

At temperatures greater than about 50° C., the silver or mercuryexchanged cationic substrate may tend to release small amounts of silveror mercury on the order of 500 wppb or less and thus the silver ormercury exchanged substrate is chemically stable under the conditions ofinterest. More preferably, silver losses are less than 100 wppb into theorganic medium and still more preferably less than 20 wppb into theorganic medium. Silver losses may be slightly higher upon start up. Inany event, if so desired a bed of acid form cationic material may beplaced downstream of the silver or mercury exchange material in additionto the bed of acid form cationic material upstream of the silver ormercury exchange material, to catch any silver or mercury released.

The pressures during the contacting steps with the exchange resins arelimited only by the physical strength of the resins. In one embodiment,the contacting is conducted at pressures ranging from 0.1 MPa to 1 MPa,e.g., from 0.1 MPa to 0.8 MPa or from 0.1 MPa to 0.5 MPa. Forconvenience, however, both pressure and temperature preferably may beestablished so that the contaminated carboxylic acid stream is processedas a liquid. Thus, for example, when operating at atmospheric pressure,which is generally preferred based on economic considerations, thetemperature may range from 17° C. (the freezing point of acetic acid) to118° C. (the boiling point of acetic acid). It is within the purview ofthose skilled in the art to determine analogous ranges for productstreams comprising other carboxylic acid compounds. The temperature ofthe contacting step preferably is kept low enough to minimize resindegradation. In one embodiment, the contacting is conducted at atemperature ranging from 25° C. to 120° C., e.g., from 25° C. to 100° C.or from 50° C. to 100° C. Some cationic macroreticular resins typicallybegin significant degrading (via the mechanism of acid-catalyzedaromatic desulfonation) at temperatures of 150° C. Carboxylic acidshaving up to 5 carbon atoms, e.g., up to 4 carbon atoms, or up to 3carbon atoms, remain liquid at these temperatures. Thus, the temperatureduring the contacting should be maintained below the degradationtemperature of the resin utilized. In some embodiments, the operatingtemperature is kept below temperature limit of the resin, consistentwith liquid phase operation and the desired kinetics for lithium and/orhalide removal.

The configuration of the resin beds within an acetic acid purificationtrain may vary, but the cationic exchanger should be upstream of themetal-exchanged resin. In a preferred embodiment, the resin beds areconfigured after a drying column. Preferably the resin beds areconfigured in a position wherein the temperature of the product streamis low, e.g., less than 120° C. or less than 100° C. The streamcontacting the acid-form cationic exchange resin and the streamcontacting the metal-exchanged resin can be adjusted to the same ordifferent temperatures. For example, the stream contacting the acid-formcationic exchange resin can be adjusted to a temperature from 25° C. to120° C., e.g., 30° C. to 100° C., 25° C. to 85° C., 40° C. to 70° C.,e.g., 40° C. to 60° C., while the stream contacting the metal-exchangedresin can be adjusted to a temperature from 50° C. to 100° C., e.g.,from 50° C. to 85° C., from 55° C. to 75° C., or from 60° C. to 70° C.Aside from the advantages discussed above, lower temperature operationprovides for less corrosion as compared to higher temperature operation.Lower temperature operation provides for less formation of corrosionmetal contaminants, which, as discussed above, may decrease overallresin life. Also, because lower operating temperatures result in lesscorrosion, vessels advantageously need not be made from expensivecorrosion-resistant metals, and lower grade metals, e.g., standardstainless steel, may be used.

Referring back to FIG. 1, product stream 146 is first passed throughcationic exchange resin bed 180 to hydrate acetic anhydride. Althoughone cationic exchange resin bed 180 is shown, it should be understoodthat a plurality of cationic exchange resin beds may be used in seriesor parallel. In some embodiments, the cationic exchangers may alsoremove other cations present in the stream, such as lithium orpotassium, if added via line 160 to drying column 130 as a potassiumsalt selected from the group consisting of potassium acetate, potassiumcarbonate, and potassium hydroxide, and corrosion metals. Using thecationic exchangers of the present invention, the purified acetic acidproduct comprises less acetic anhydride than the product stream.

The resulting purified acetic acid 182 may passes through ametal-exchanged ion exchange resin bed 184 having acid cation exchangesites comprising at least one metal selected from the group consistingof silver, mercury, palladium and rhodium to remove iodides from thestream to produce a purified product 186. Although one metal-exchangedion exchange resin bed 184 is shown, it should be understood that aplurality of metal-exchanged ion exchange resin beds may be used inseries or parallel. In addition to the resin beds, heat exchangers (notshown) may be located before either resin bed to adjust the temperatureof the stream 146 and 182 to the appropriate temperature beforecontacting the resin beds. Similarly in FIG. 2, the crude acetic acidproduct is fed to cationic exchange resin bed 180 from side stream 170.Heat exchangers or condensers may be located before either resin bed toadjust the temperature of the stream 170 to the appropriate temperaturebefore contacting the resin beds.

In one embodiment, the flow rate through the resin beds ranges from 0.1bed volumes per hour (“BV/hr”) to 50 BV/hr, e.g., 1 BV/hr to 20 BV/hr orfrom 6 BV/hr to 10 BV/hr. A bed volume of organic medium is a volume ofthe medium equal to the volume occupied by the resin bed. A flow rate of1 BV/hr means that a quantity of organic liquid equal to the volumeoccupied by the resin bed passes through the resin bed in a one hourtime period.

A purified acetic acid composition is obtained as a result of the resinbed treatment. The purified acetic acid composition comprises aceticanhydride at a concentration of no more than 50 wppm, e.g., no more than40 wppm, no more than 30 wppm, no more than 20 wppm, no more than 10wppm or no more than 5 wppm. In terms of ranges, the purified aceticacid product comprises acetic anhydride at a concentration from 0.5 to50 wppm, e.g., from 0.5 to 40 wppm, from 0.5 to 30 wppm, from 0.5 to 20wppm, or from 0.5 to 10 wppm. The purified acetic acid product comprisesless acetic anhydride than the product stream. The purified acetic acidcomposition, in one embodiment, comprises iodides in an amount of nomore than 100 wppb, e.g., no more than 90 wppb, no more than 50 wppb, nomore than 25 wppb, or no more than 15 wppb. In one embodiment, thepurified acetic acid composition comprises lithium in an amount of nomore than 100 wppb, e.g., no more than 50 wppb, no more than 20 wppb, orno more than 10 wppb. In terms of ranges, the purified acetic acidcomposition may comprise from 0 to 100 wppb iodides, e.g., from 0 to 50wppb, from 1 to 50 wppb, from 2 to 40 wppb; and/or from 0 to 100 wppblithium, e.g., from 1 to 50 wppb, from 2 to 40 wppb. In otherembodiments, the resin beds remove at least 25 wt. % of the iodides fromthe product stream, e.g., at least 50 wt. % or at least 75 wt. %.

In addition to reducing acetic anhydride, the present invention may alsoreduce a metal displaced from the metal-exchanged ion exchange resin,e.g. silver, mercury, palladium and rhodium, that undesirably accumulatein the purified acetic acid as the final product when no cationicexchanger is used to remove cations, such as lithium derived from and/orgenerated by the lithium compound in the reaction medium. In oneembodiment, the purified acetic acid comprises a metal displaced fromthe metal-exchanged ion exchange resin, e.g., silver, mercury, palladiumand rhodium, in an amount of no more than 100 wppb, e.g., no more than90 wppb, no more than 80 wppb, no more than 70 wppb, no more than 60wppb, no more than 50 wppb, no more than 40 wppb, no more than 30 wppb,or no more than 20 wppb. In terms of ranges, the purified acetic acidcomprises a metal displaced from the metal-exchanged ion exchange resin,e.g., silver, mercury, palladium and rhodium, in an amount from 0 to 100wppb, e.g., from 0.1 to 100 wppb, from 0.5 to 90 wppb, from 1 to 80wppb, from 1 to 70 wppb, from 1 to 60 wppb, from 1 to 50 wppb, from 1 to40 wppb, from 1 to 30 wppb, or from 1 to 20 wppb.

Distillation

The distillation columns of the present invention may be a conventionaldistillation column, e.g., a plate column, a packed column, and others.Plate columns may include a perforated plate column, bubble-cap column,Kittel tray column, uniflux tray, or a ripple tray column. For a platecolumn, the theoretical number of plates is not particularly limited anddepending on the species of the component to be separate, may include upto 80 plates, e.g., from 2 to 80, from 5 to 60, from 5 to 50, or morepreferably from 7 to 35. The distillation column may include acombination of different distillation apparatuses. For example, acombination of bubble-cap column and perforated plate column may be usedas well as a combination of perforated plate column and a packed column.

The distillation temperature and pressure in the distillation system cansuitably be selected depending on the condition such as the species ofthe objective carboxylic acid and the species of the distillationcolumn, or the removal target selected from the lower boiling pointimpurity and the higher boiling point impurity according to thecomposition of the feed stream. For example, in a case where thepurification of acetic acid is carried out by the distillation column,the inner pressure of the distillation column (usually, the pressure ofthe column top) may be from 0.01 to 1 MPa, e.g., from 0.02 to 0.7 MPa,and more preferably from 0.05 to 0.5 MPa in terms of gauge pressure.Moreover, the distillation temperature for the distillation column,namely the inner temperature of the column at the temperature of thecolumn top, can be controlled by adjusting the inner pressure of thecolumn, and, for example, may be from 20 to 200° C., e.g., from 50 to180° C., and more preferably from 100 to 160° C.

The material of each member or unit associated with the distillationsystem, including the columns, valves, condensers, receivers, pumps,reboilers, and internals, and various lines, each communicating to thedistillation system may be made of suitable materials such as glass,metal, ceramic, or combinations thereof, and is not particularly limitedto a specific one. According to the present invention, the material ofthe foregoing distillation system and various lines are a transitionmetal or a transition-metal-based alloy such as iron alloy, e.g., astainless steel, nickel or nickel alloy, zirconium or zirconium alloythereof, titanium or titanium alloy thereof, or aluminum alloy. Suitableiron-based alloys include those containing iron as a main component,e.g., a stainless steel that also comprises chromium, nickel, molybdenumand others. Suitable nickel-based alloys include those alloys containingnickel as a main component and one or more of chromium, iron, cobalt,molybdenum, tungsten, manganese, and others, e.g., HASTELLOY™ andINCONEL™. Corrosion-resistant metals may be particularly suitable asmaterials for the distillation system and various lines.

As is evident from the figures and text presented above, a variety ofembodiments are contemplated.

The present invention will be better understood in view of the followingnon-limiting examples.

EXAMPLES Example 1

A stream comprising acetic acid, less than 2 wt. % water, and aceticanhydride that varied from 123 to 510 ppm was fed to a column filledwith Ag functionalized sulfonic resin at 8 bed volume per hour flowrate. The temperature of the resin was controlled at 75° C. The aceticanhydride concentration in the inlet and outlet of the resin column weremeasured by a gas chromatograph (GC) equipped with a flame ionizationdetector. A capillary column with dimethylpolysiloxane stationary phasewas used to achieve separation in the GC analysis. Results forexperiments 1-3 are shown in Table 4.

TABLE 4 Flow Through Experiments Performed at 75° C. Experiment No.Inlet Conc., ppm Outlet Conc., ppm 1 123 2 2 235 2 3 510 1

Example 2

Experiments 4-6 were performed with the same procedures and setup asExample 1 except the initial acetic anhydride concentration varied from171 to 574 ppm and the resin column temperature that was controlled at25° C. Results are shown in Table 5.

TABLE 5 Flow Through Experiments Performed at 25° C. Experiment No.Inlet Conc., ppm Outlet Conc., ppm 4 171 1 5 333 1 6 574 2

While the invention has been described in detail, modifications withinthe spirit and scope of the invention will be readily apparent to thoseof skill in the art. In view of the foregoing discussion, relevantknowledge in the art and references discussed above in connection withthe Background and Detailed Description, the disclosures of which areall incorporated herein by reference. In addition, it should beunderstood that aspects of the invention and portions of variousembodiments and various features recited below and/or in the appendedclaims may be combined or interchanged either in whole or in part. Inthe foregoing descriptions of the various embodiments, those embodimentswhich refer to another embodiment may be appropriately combined withother embodiments as will be appreciated by one of skill in the art.Furthermore, those of ordinary skill in the art will appreciate that theforegoing description is by way of example only, and is not intended tolimit the invention.

We claim:
 1. A process for purifying acetic acid, comprising: a.distilling a process stream in a column, the process stream comprisingacetic acid at a concentration of greater than 90 wt. %, water at aconcentration from 1 to 3 wt. %, one or more C₁-C₁₄ alkyl iodides in atotal concentration of no more than 6 wt. %, and methyl acetate at aconcentration of no more than 6 wt. %; b. forming acetic anhydride in alower portion of the column; c. withdrawing a product stream from thelower portion of the column, the product stream comprising acetic acid,water at a concentration of no more than 0.2 wt. %, and acetic anhydrideat a concentration of no more than 600 wppm; and d. hydrating the aceticanhydride in the product stream to form a purified acetic acid productcomprising acetic anhydride at a concentration of no more than 50 wppm.2. The process of claim 1, wherein the hydrating step comprisescontacting the product stream with a cation exchange resin.
 3. Theprocess of claim 2, wherein the cation exchange resin comprises sulfonicacid or partially metal impregnated sulfonic acid functional groups. 4.The process of claim 1, wherein the product stream is not contacted withan aqueous stream after withdrawal of the product stream from the lowerportion of the column.
 5. The process of claim 1, wherein the processstream is substantially free of acetic anhydride.
 6. The process ofclaim 1, wherein the process stream comprises hydrogen iodide at aconcentration of no more than 300 wppm.
 7. The process of claim 1,wherein the product stream comprises acetic anhydride at a concentrationfrom 5 to 600 wppm.
 8. The process of claim 1, wherein the purifiedacetic acid product comprises water at a concentration of no more than0.2 wt. %.
 9. The process of claim 1, wherein the purified acetic acidproduct comprises acetic anhydride at a concentration from 0.5 to 50wppm.
 10. The process of claim 1, wherein the product stream iswithdrawn at a point within 5 actual stages from the base of the column.11. The process of claim 1, further comprising contacting the purifiedacetic acid product with a metal-exchanged ion exchange resin havingacid cation exchange sites to reduce the iodide concentration of thepurified acetic acid product to no more than 100 wppb.
 12. A process forproducing acetic acid comprising: a. carbonylating, in a reactor, atleast one member selected from the group consisting of methanol,dimethyl ether, and methyl acetate in a reaction medium comprising waterat a concentration from 0.1 to 14 wt. %, a rhodium catalyst, methyliodide, and an iodide salt; b. separating the reaction medium to form aliquid recycle stream and a vapor product stream; c. distilling at leasta portion of the vapor product stream in a first column to obtain a sidestream comprising acetic acid at a concentration greater than 90 wt. %,water at a concentration from 1 to 3 wt. %, one or more C₁-C₁₄ alkyliodides in a total concentration of no more than 6 wt. % and methylacetate at a concentration of no more than 6 wt. %; d. distilling theside stream in a second column to obtain a product stream comprisingacetic acid, water at a concentration of no more than 0.2 wt. % andacetic anhydride at a concentration of no more than 600 wppm; and e.contacting the product stream with a cation exchange resin to form apurified acetic acid product comprising no more than 50 wppm aceticanhydride.
 13. The process of claim 12, wherein the production rate ofacetic anhydride is greater in the second column than the reactor. 14.The process of claim 12, wherein the side stream is substantially freeof acetic anhydride.
 15. The process of claim 12, wherein the sidestream comprises hydrogen iodide at a concentration of no more than 300wppm.
 16. The process of claim 12, wherein the product stream comprisesacetic anhydride at a concentration from 10 to 600 wppm.
 17. The processof claim 12, wherein the purified acetic acid product comprises water ata concentration of no more than 0.2 wt. %.
 18. The process of claim 12,wherein the purified acetic acid product comprises acetic anhydride at aconcentration of no more than 10 wppm.
 19. The process of claim 12,wherein the acetic anhydride concentration of the purified acetic acidproduct is less than the acetic anhydride concentration of the productstream.
 20. The process of claim 12, further comprising contacting thepurified acetic acid product with a metal-exchanged ion exchange resinhaving acid cation exchange sites to reduce the iodide concentration ofthe purified acetic acid product to no more than 100 wppb.